The invention relates to tensioners, and more particularly, to tensioners having a constant tension for a range of linear movement.
Most engines used for automobiles and the like include a number of belt driven accessory systems which are necessary for the proper operation of the engine. The accessory systems may include an alternator, air conditioner compressor and a power steering pump.
The accessory systems are generally mounted on a front surface of the engine. Each accessory having a pulley mounted on a shaft for receiving power from some form of belt drive. In early systems, each accessory was driven by a dedicated belt that ran between the accessory and the crankshaft. With improvements in belt technology, single serpentine belts are now used in most applications, routed among the various accessory components. The serpentine belt is driven by the engine crankshaft.
Since the serpentine belt must be routed to all accessories, it has generally become longer than its predecessors. To operate properly, the belt is installed with a pre-determined tension. As it operates, it stretches slightly. This results in a decrease in belt tension, which may cause the belt to slip, causing undue noise and wear. Consequently, a belt tensioner is desirable to maintain the proper belt tension as the belt stretches during use.
As a belt tensioner operates, the belt usually oscillates due to its interaction with the pulleys. These oscillations are undesirable, as they cause premature wear of the belt and tensioner. Therefore, a damping mechanism is added to the tensioner to damp the belt oscillations.
The prior art tensioners rely on some sort of loading element to operate on a pulley. These comprise compression springs, shock absorbers, air springs, hydraulic cylinders, and so on. In this case the belt tension and consequently the load on the pulley is a function of its position. Generally, the tension in the belt increases or decreases depending on the position of the pulley.
In addition, various prior art damping mechanisms have been developed. They include viscous fluid based dampers, mechanisms based on frictional surfaces sliding or interaction with each other, and dampers using a series of interacting springs. Each relies on a single form of damping mechanism to perform the damping function. Each has a pulley and damping mechanism configuration with the damping mechanism external to the pulley. This created an unduly large device for the purpose.
What is needed is a tensioner having a constant tension over a range of linear motion of a pulley. What is needed is a tensioner having a camming body to provide a constant force over a range of spring torque""s. What is needed is a tensioner having a damping mechanism comprising a linear guide. The present invention meets these needs.
The primary aspect of the invention is to provide a tensioner having a constant tension over a range of linear motion of a pulley.
Another aspect of the invention is to provide a tensioner having a camming body to provide a constant force over a range of spring torque""s.
Another aspect of the invention is to provide a tensioner having a damping mechanism comprising a linear guide.
Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings.
The invention comprises a tensioner having a pulley journaled to a housing. The housing further comprises a guide. The guide slidingly engages rails on a base. The guide and rails constrain the housing to move in a predetermined linear path. The rails are horizontally offset from two axis defined by the pulley. The pulley load is also vertically offset from the axis of the guide. The guide and rails have a predetermined frictional coefficient so that the cumulative result is an asymmetric damping effect. Further, a linkage is connected between the housing and a camming body. The camming body is rotatably connected to the base. A biasing member such as a torsion spring biases the camming body against the belt load through the linkage. The radius of the camming body is variable to maintain a constant belt load as the tensioner pulley moves in response to a load change.